The sophistication of internal combustion engines and engine operating schemes continues to advance, and innovations in design and operation are revealed regularly. One driving force behind many of the design changes in recent years have been increasingly stringent engine emissions requirements. One general approach to improving emissions quality relates to treatment of combustion products downstream from the engine. In other words, exhaust gases produced by the engine are treated via a variety of chemical and/or physical processes in an attempt to remove, alter or reduce undesired constituents. Other engine developers have focused more on the combustion process itself. Manipulation of fuel injection quantity, frequency, timing and even the type of fuel spray pattern has been shown to have various effects on engine emissions. Of particular interest to engineers are increasingly stringent jurisdictional standards relating to emission limits on various nitrogen-oxygen compounds, known collectively as “NOx”.
It has been discovered that enhancing mixing of air and fuel prior to ignition in an internal combustion engine cylinder can help reduce NOx levels in the engine exhaust. One approach in particular is known in the art as “homogeneous charge” compression ignition. In the compression ignition engine arts, this approach is widely referred to as “HCCI”. In a homogeneous charge mode, fuel may be injected into a compression ignition engine cylinder prior to the point during an engine cycle at which cylinder conditions will trigger autoignition. This differs from a more traditional approach, wherein fuel is primarily injected during an engine cycle at or close to a point at which autoignition can occur. In other words, rather than fuel more or less continuously combusting as it leaves the fuel injector tip, in homogeneous charge mode the fuel may be injected in advance of autoignition conditions, such that the fuel and air have relatively more time to mix as the piston travels upward in the cylinder.
Homogeneous charge operation tends to be relatively sensitive to various operating conditions external to and internal of the engine. Ambient temperature and pressure, as well as the timing of autoignition conditions in the engine cycle, for example, can affect the ability of an engine to successfully operate in a homogeneous charge mode. In particular, the combustion phasing of HCCI charges tends to vary in response to varying operating conditions.
In certain instances, it may be desirable to operate an engine in a mixed homogeneous charge and conventional mode, employing both HCCI fuel injections and conventional fuel injections during operation. Some operating schemes actually utilize both injection types in the same engine cycle. Where multiple fuel injections are delivered in the same engine cycle, control over the appropriate combustion phasing of the respective charges becomes even more difficult.
U.S. Pat. No. 6,668,789 to Marriott et al. is directed to one type of HCCI operating scheme stated to obviate certain problems relating to combustion event timing. In particular, Marriott employs multiple injections during a piston compression stroke, with the second injected charge serving as an ignition means for the first injected charge. While Marriott appears to offer advantages in certain operating environments, there is always room for improvement.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.